One disadvantage of prior art acoustic scanning systems has been a lack of capability of such systems to be operated effectively at very high frequencies. A very high frequency (small wavelength) is desirable because the smallest beam cross-section dimension which can be theoretically attained is proportionally related to the wavelength. Image resolution is limited by the size of this dimension.
Prior art U.S. Pat. No. 4,011,747 generates scanning focused acoustic bulk waves by launching chirp surface acoustic waves along a surface of a solid medium and scattering the propagating surface waves into bulk waves using an acoustic grating on the surface. The scattered acoustic bulk waves are automatically focused as a result of the chirp and scan at substantially the same speed as the surface acoustic waves propagate.
One disadvantage of this prior art arrangement is that the propagating surface waves gradually lose amplitude as they travel across the grating, which results in a scanning acoustic wave which loses amplitude during a scan. In theory, this effect may be compensated by using a grating with grooves of gradually increasing depth. However, such gratings are difficult and expensive to fabricate.
Another disadvantage is that the high frequency performance of this device is compromised by undesirable characteristics of the materials available for the solid medium, which must simultaneously allow both efficient generation and low loss propagation of surface acoustic waves at high frequencies. Materials which allow reasonably efficient generation and propagation of surface acoustic waves at high frequencies are piezoelectric. Unfortunately these materials are anisotropic, so that the ability to focus an acoustic bulk wave is adversely affected. Since the chief reason for using a higher frequency is to reduce one or both of the cross-section dimensions of the acoustic bulk beam, any loss in ability to focus reduces or eliminates the advantage.
Piezoelectric materials also have a much higher surface acoustic wave propagation velocity than some non-piezoelectric materials. All else being equal, this results in a larger minimum dimension for the beam. The frequency must be raised still more to compensate for the higher propagation velocity. Since the beam scanning speed is substantially equal to the surface wave propagation velocity, a higher propagation velocity also increases some of the performance requirements for electronic components.
Use of a piezoelectric solid medium in the prior are device has further disadvantages relating to impedance matching. The acoustic bulk beam must leave the solid medium in order to reach a target. The high characteristic acoustic impedance of piezoelectric materials results in very great amplitude losses and reflections at the material interfaces unless a plurality of special impedance matching layers are used.
Surface acoustic wave propagation is very sensitive to surface boundary conditions. Therefore, the acoustic grating surface in the prior art device cannot be contaminated or placed in contact with any foreign body or substance, which is a further disadvantage.
It is the primary object of this invention to provide apparatus and a method for generating focused acoustic bulk waves which overcome the disadvantages and limitations of the described prior art.
One object is to generate focused acoustic bulk waves at higher acoustic frequencies.
Another object is to generate scanning focused acoustic bulk waves using a uniform depth acoustic grating and not have the focused acoustic bulk waves lose amplitude during a scan.
It is also an object to generate scanning focused acoustic bulk waves at very high frequencies without using piezoelectric or other anisotropic materials.
A further object is to more efficiently generate scanning focused acoustic bulk waves, more efficiently propagate them, and more efficiently transfer them into liquid media such as water.
Still another object is to avoid use of surface acoustic waves and acoustic gratings which interact with surface acoustic waves so that there will be no contamination problem or need to avoid contact with any of the device surfaces.
These and further objects are achieved by the invention.